Technical Abstract:
Huanglongbing (HLB, yellow shoot disease) is a highly destructive disease that threatens citrus production worldwide. The disease was first observed in Guangdong, P. R. China over 100 years ago, and was found in Florida, USA in 2005. “Candidatus Liberibacter asiaticus” has been associated with HLB. Understanding the global epidemiology of HLB is important for management of the disease. In this study, we identified a genetic marker containing small tandem repeats in the genome of “Ca. L. asiaticus” and comparatively analyzed the tandem repeat numbers (TRNs) in bacterial populations from Guangdong and Florida. The Guangdong population consisted predominately of strains with TRN of 7 (TRN7) at a frequency of 47.6%. The Florida population consisted predominately of strains with TRN of 5 (TRN5) at a frequency of 84.4%. TRNs ranged from 3 to 16. The apparent absence of TRN of 9, 10, 11, and 12 separated the bacterial strains into two groups: TRN less than 10 (TRN<10) and TRN greater than 10 (TRN>10). In Florida, TRN<10 strains (103/109 or 94.5%) were widely distributed in all HLB-affected counties. TRN>10 strains (6/109 or 5.5%) were found in central Florida. This is the first report documenting the differentiation of “Ca. L. asiaticus” populations between Asia and North America and the possible presence of two differentially distributed genotypes of “Ca. L. asiaticus” in Florida.